Drill stem testing (herein referred to as DST) is a common feature in the exploration for oil or gas. When analysed correctly, the test may be used for the calculation of permeability and reservoir pressure. In its traditional form, DST involves fining a valve and an expandable packer below the drill string which is then run into a borehole with the drill string empty. A packer is traditionally set to seal the borehole by applying compression to it via an extension which rests on the borehole base. A valve is opened by rotating the drill string so that fluid from the formation below the packer flows through the packer and the valve into the drill stem. After a flow period, the valve is closed by further rotation of the drill string. The pressure in the borehole may then build up. The downhole pressure is usually recorded by a memory gauge, or the like. Flow may be recorded at the surface or by a second memory gauge which records fluid level changes within the drill pipe.
In more advanced forms, DST can use a straddle packer arrangement to seal the borehole above and below a test zone. In this case, the packers used are usually inflatable units which are inflated by a downhole pump. This may be activated by drill string rotation or by electrical means. Some tools use a form of radio telemetry to transmit information to the surface concerning pressures measured during the test.
Some tools also exist which are specifically designed to undertake injection rather than production, as in DST. Some of these may be run on the end of a drill string and are normally inflated by pressurising the drill string before opening a valve to inject fluid into the formation. Some variants of these toots may be used in conjunction with wireline coring systems. In this case, the tool is generally lowered through the drill string to seat in place of the inner core barrel, but with packer(s) protruding through the end of the core barrel so as to seal against the borehole.